simplify flattener api

internal/cmdrootfs/cmdrootfs.go

@@ -13,11 +13,6 @@ import (
 )
 
 type (
-	flattener interface {
-		Flatten(io.Writer) error
-	}
-
-	rootfsFactory func(io.ReadSeeker) flattener
 
 	// Command is "rootfs" command
 	Command struct {
@@ -28,7 +23,7 @@ type (
 			Outfile string           `long:"outfile" description:"Output path, stdout is '-'"`
 		} `positional-args:"yes" required:"yes"`
 
-		rootfsNew rootfsFactory
+		flattener func(io.ReadSeeker, io.Writer) error
 	}
 )
 
@@ -37,7 +32,7 @@ func (c *Command) Execute(args []string) (err error) {
 	if len(args) != 0 {
 		return errors.New("too many args")
 	}
-	if c.rootfsNew == nil {
+	if c.flattener == nil {
 		c.init()
 	}
 
@@ -59,7 +54,7 @@ func (c *Command) Execute(args []string) (err error) {
 		out = outf
 	}
 
-	return c.rootfsNew(rd).Flatten(out)
+	return c.flattener(rd, out)
 }
 
 // init() initializes Command with the default options.
@@ -68,7 +63,5 @@ func (c *Command) Execute(args []string) (err error) {
 // command will initialize itself.
 func (c *Command) init() {
 	c.BaseCommand.Init()
-	c.rootfsNew = func(r io.ReadSeeker) flattener {
-		return rootfs.New(r)
-	}
+	c.flattener = rootfs.Flatten
 }

internal/cmdrootfs/cmdrootfs_test.go

@@ -67,9 +67,7 @@ func TestExecute(t *testing.T) {
 			inf := filepath.Join(dir, tt.infile)
 			c.PositionalArgs.Infile = goflags.Filename(inf)
 			c.PositionalArgs.Outfile = tt.outfile
-			c.rootfsNew = func(r io.ReadSeeker) flattener {
-				return &passthrough{r}
-			}
+			c.flattener = flattenPassthrough
 
 			err := c.Execute(nil)
 			if tt.wantErr != "" {
@@ -90,9 +88,7 @@ func TestExecute(t *testing.T) {
 	}
 }
 
-type passthrough struct{ r io.Reader }
-
-func (p *passthrough) Flatten(w io.Writer) error {
-	_, err := io.Copy(w, p.r)
+func flattenPassthrough(r io.ReadSeeker, w io.Writer) error {
+	_, err := io.Copy(w, r)
 	return err
 }

lxcconfig/lxcconfig_test.go

@@ -13,7 +13,6 @@ import (
 )
 
 func TestLXCConfig(t *testing.T) {
-
 	tests := []struct {
 		name   string
 		docker dockerConfig

rootfs/rootfs.go

@@ -27,11 +27,6 @@ var (
 )
 
 type (
-	// RootFS accepts a docker layer tarball and flattens it.
-	RootFS struct {
-		rd io.ReadSeeker
-	}
-
 	dockerManifestJSON []struct {
 		Layers []string `json:"Layers"`
 	}
@@ -42,14 +37,11 @@ type (
 	}
 )
 
-// New creates a new RootFS'er.
-func New(rd io.ReadSeeker) *RootFS {
-	return &RootFS{rd: rd}
-}
-
-// Flatten flattens a docker image to an open tarball.
-func (r *RootFS) Flatten(w io.Writer) (err error) {
-	tr := tar.NewReader(r.rd)
+// Flatten flattens a docker image to a tarball. The underlying io.Writer
+// should be an open file handle, which the caller is responsible for closing
+// themselves
+func Flatten(rd io.ReadSeeker, w io.Writer) (err error) {
+	tr := tar.NewReader(rd)
 	var closer func() error
 
 	// layerOffsets maps a layer name (a9b123c0daa/layer.tar) to it's offset
@@ -74,7 +66,7 @@ func (r *RootFS) Flatten(w io.Writer) (err error) {
 				return fmt.Errorf("decode %s: %w", _manifestJSON, err)
 			}
 		case strings.HasSuffix(hdr.Name, _layerSuffix):
-			here, err := r.rd.Seek(0, io.SeekCurrent)
+			here, err := rd.Seek(0, io.SeekCurrent)
 			if err != nil {
 				return err
 			}
@@ -107,10 +99,10 @@ func (r *RootFS) Flatten(w io.Writer) (err error) {
 
 	// iterate over all files, construct `file2layer`, `whreaddir`, `wh`
 	for i, no := range layers {
-		if _, err := r.rd.Seek(no.offset, io.SeekStart); err != nil {
+		if _, err := rd.Seek(no.offset, io.SeekStart); err != nil {
 			return err
 		}
-		tr, closer = openTargz(r.rd)
+		tr, closer = openTargz(rd)
 		for {
 			hdr, err := tr.Next()
 			if err == io.EOF {
@@ -154,10 +146,10 @@ func (r *RootFS) Flatten(w io.Writer) (err error) {
 	}()
 	// iterate through all layers, all files, and write files.
 	for i, no := range layers {
-		if _, err := r.rd.Seek(no.offset, io.SeekStart); err != nil {
+		if _, err := rd.Seek(no.offset, io.SeekStart); err != nil {
 			return err
 		}
-		tr, closer = openTargz(r.rd)
+		tr, closer = openTargz(rd)
 		for {
 			hdr, err := tr.Next()
 			if err == io.EOF {

rootfs/rootfs_test.go

@@ -186,7 +186,7 @@ func TestRootFS(t *testing.T) {
 			in := bytes.NewReader(tt.image.Buffer().Bytes())
 			out := bytes.Buffer{}
 
-			err := New(in).Flatten(&out)
+			err := Flatten(in, &out)
 			if tt.wantErr != "" {
 				assert.EqualError(t, err, tt.wantErr)
 				return